Japanese Patent Application Publication No. Hei 5-30608 discloses an injection molding apparatus for molding a plastic spectacle lens. In the inside of an injection molding die in the injection molding apparatus, two cavities formed with tubular insert guides and inserts slidably inserted into the insert guides, a gate opened toward each cavity as an inlet port of a molten resin into the cavity, a runner connecting between the cavities through the gate, and a sprue connected to the central portion of the runner at the right angle are provided.
In order to mold the lens, first, the volume of the cavity is increased by moving the insert back in the insert guide. In this state, the molten resin is injected through an injection nozzle into the sprue. The molten resin injected into the sprue is fed through the runner and the gate into the cavity, and after feeding, the molten resin in the cavity is compressedly molded by moving the insert forward in the insert guide.
As a result, the plastic spectacle lens, having plural lens portions compressedly molded in the cavities, a runner forming portion formed by the runner to connect between the lens portions, and a sprue forming portion formed by the sprue to be connected to the runner forming portion, is obtained.
After then, the plastic spectacle lens molding undergoes a coating process with a coating fluid after a washing process with a cleaning fluid as after-treatment, thereby sufficient durability of the lens surface for the spectacle lens is obtained.
Various spectacle lenses can be molded in the aforementioned injection molding die by exchanging the inserts. The spectacle lens has various kinds of lens, such as a plus lens having the thinner peripheral portion than the central portion, a minus lens having the thicker peripheral portion than the central portion, a semi-finish lens having the sufficient thickness for preconditioning a following process for a concave surface of lens or a convex surface of lens and various lenses having the different diopter of the above lenses.
The spectacle lens having the diopter range of +4.00 Diopter to -8.00 is preferably used for the molding method.
The thickness of the lens is different dependent upon a design of an optical lens, for example, an aspherical lens, having approximate 1.50 of index of refraction, can be adjusted to designing values of 4.2 mm of the thickness of the central portion and 1.0 mm of the thickness of the peripheral portion in the case of +2.00 Diopter, and of 1.4 mm of the thickness of the central portion and 7.9 mm of the thickness of the peripheral portion in the case of 4.00 Diopter. However, when the above lenses are molded to be compressed under the same conditions, the following disadvantages are produced by the difference of a shape properties.
For example, in the molding of the minus lens, the central portion of the lens is thinner than the peripheral portion thereof, so that the central portion of the cavity has a large flow-resistance. Therefore, the molten resin injected into the cavity hardly flows in the central portion of the cavity, so that the resin flows around from the peripheral portion to the central portion by separately flowing, resulting in a disadvantage that a lot of weld marks are easily produced in the central portion.
Further, in the molding of the plus lens, when the difference between the volume of the cavity and the quantity of the fed molten resin is extremely big, the flow property of the molten resin is reduced, and naturally, a non-fed part is produced, with the result that a flow mark is easily produced on the lens after molding
The spectacle lens is a precision molding, therefore, in order to mold each lens with a high power of precision, it is important that the molten resin is evenly and reliably fed into every corner of the cavity without remaining a feed history of the resin of the aforementioned weld mark, the flow mark and so on to correspond with each of various lenses. Therefore, a structure of the gate as an inlet port of the molten resin into the cavity is critical. As the gate structure having an opening area capable of ensuring the specified amount of feeding the molten resin for a specified time, for example Japanese Patent Application Publication No. Hei 5-44893 discloses.
However, the injection method for the lens having a different thickness between the central portion and the peripheral portion like a spectacle lens is difficult, especially, the plus lens has the thinner peripheral portion, so that a change in configuration including polymerization contraction, known as "molding sink", is produced adjacent to the gate, therefore the precision of the configuration of the peripheral portion is hardly retained. The gate opening configuration relates to the flow properties of the fed resin, so that the inferior feeding or the flow mark are easily produced in dependence on the gate opening configuration, for example, in the case the minus lens is molded, when the resin is not smoothly flown into the cavity, the weld mark has a propensity to be noticeably produced. Furthermore, for example, in the case of typical polycarbonate resin, excessive back-flow of the resin injected into the cavity once becomes a cause of easily producing a cut and deformation and a uneven flow history of the resin easily leaves an injection history on the surface of the lens. Therefore, the gate structure capable of obtaining the appropriate back-flow and the flow properties of the resin has been required.
In the case that the lens portion formed in the cavity for a molding is a plus lens having a meniscus-shape of the thicker central portion of the lens than the peripheral portion, it is thought that the injection molding is carried out by decreasing the size of the gate opening as a connection portion of the cavity and the runner. The reason is, as described above, that the back-flow in the peripheral portion is controlled and "molding sink" is hardly produced, because "molding sink" has the effect on the configuration accuracy of the thinner peripheral portion when the molten resin in the two cavities are compressedly molded with the movement (forward) of the insert for the insert guide.
When the gate opening is to be smaller, the back-flow is hardly produced in the compression molding by slidably moving the insert, but a void space as a lacked portion caused by shrinkage of the molten resin, which is different from "molding sink", is easily produced at the connection portion of the runner and the sprue, which has increased volume.
If the void space is produced on the plastic spectacle lens molding, in the washing process with the cleaning fluid and the coating process with the coating fluid as described above, the cleaning fluid remains in the concave void space, a white turbidity fluid is produced by reaction with the cleaning fluid remaining the void space and the coating fluid including an organosilicon compound and/or its hydrolysate, and the surface of the lens portion is stained with the white turbidity fluid, resulting in loss of commercial value of the spectacle lens.
As to the disadvantages described thus far, in the plus lens having a low-power lens diopter, the flow properties of the molten resin is not needed to be defined at high because of the small different thickness of the central portion and the peripheral portion of the lens, so that the temperature of the injection molding die is defined at low and the back flow of the molten resin is hardly produced, and when a non-astigmatic lens is molded as compared with molding an astigmatic lens, the difference of the thickness of the central portion and the peripheral portion of lens is larger, namely, the thickness of the peripheral portion is thinner, so that the gate opening is defined to be small, resulting in noticeable occurrence of the back flow.
It is an object of the present invention to provide the lens producing method and the injection molding die for molding the lens to be able to mold an accurate lens without influence of the lens configuration including the difference of the thickness of the central portion and the peripheral portion of the lens.
It is another object of the present invention to provide the lens producing method, the injection molding die for molding the lens and the lens molding, in which the occurrence of the void space is controlled, so that the white turbidity fluid is not produced by operating the washing process with the cleaning fluid and the coating process with the coating fluid.